The present invention relates to dielectric filter devices for determining the frequency band pass characteristics to be used in mobile communications devices or the like.
It is generally known that monoblock-type dielectric filters are used in mobile communications devices for transmitting and receiving signals, for example, in the frequency band of hundreds of megahertz to several gigahertz. Mobile communications devices, such as portable telephones, in recent years are adapted to serve a multiplicity of functions and made more compact and lightweight, and many monoblock-type dielectric duplexers are singly adapted to process different transmission and receiving frequencies for use in such devices (for example, Japanese Patent No. 3205337).
An example of duplexer which is such a conventional dielectric filter of the monoblock type will be described with reference to FIG. 5. A dielectric block 10, for example, of a ceramic material or the like has hollow bores 40 to 44, 49, 50. Electrically conductive layers are formed on the outer peripheral side surface 20 of the block and the upper surface thereof around the openings of the bores as indicated at 33 to 39. Further provided on the upper surface are a first electrode 30, second electrode 31 and third electrode 32 which are separate from these conductive layers.
As shown in FIG. 5, the bores 40, 41 are arranged between the first electrode 30 and the second electrode 31. The bore 49 is positioned between the second electrode 31 and the peripheral side surface. The bores 42 to 44 are arranged between the first electrode 30 and the third electrode 32. The bore 50 is positioned between the third electrode 32 and the peripheral side surface 20. The first electrode 30 is connected to an antenna (not shown), the second electrode 31 to a transmitter, and the third electrode 32 to a receiver.
A band-pass filter for the desired transmission frequency band is provided by suitably adjusting the dimensions and shapes (e.g., the diameter and depth of bores, and distance between conductive layers) of the bores 40, 41 between the first and second electrodes 30, 31 and the conductive layers 33, 34 in the vicinity of openings of these bores. A trap having an attenuation pole at the desired frequency in the vicinity of the above-mentioned transmission frequency band can be provided by suitably adjusting the dimensions and shapes of the bore 49 between the second electrode 31 and the conductive layer on the peripheral side surface 20 and the conductive layer 38 in the vicinity of opening of the bore
Similarly, a predetermined band-pass filter for the receiving frequency band is provided by suitably adjusting the dimensions and shapes of the bores 42 to 44 between the first and third electrodes 30, 32 and the conductive layers 35 to 37 in the vicinity of openings of these bores. A trap having an attenuation pole in the vicinity of the receiving frequency band can be provided by suitably adjusting the shapes of the bore 50 between the third electrode 32 and the conductive layer on the peripheral side surface and the conductive layer 39 in the vicinity of opening of the bore.
The dielectric duplexer described comprises two dielectric filters having two systems, i.e., transmitting system and receiving system, which use different frequencies. Since the filters are based on the same principles of the band-pass function and the trap function, these functions will be described with respect to the receiving system with reference to the filter characteristics diagrams of FIGS. 7 and 8.
FIG. 8 shows the characteristics of the band-pass filter which is capacitance-coupled by the bores provided between the first electrode and the third electrode and the conductive layers in the vicinity of these bores. The shapes of these bores and the conductive layers in the vicinity of the openings thereof are so adjusted as to provide the desired frequency pass band f1 to f2. Ideally, it is desirable that the attenuation be as great as possible in the suppressing band outside the pass band, whereas in actuality, a sharp attenuation is not available at a frequency in the suppressing band near the pass band.
In communications devices, however, it is likely that a frequency near the pass band will also be used, for example, for processing signals in the receiver provided subsequent to the filter. In such a case, the receiving filter needs to have such attenuation characteristics that in the range of from the frequency f2 at the pass-band limit to a frequency f4 which is used for other signal transmission or receiving, the attenuation is at least a (dB) for example at a frequency of f3 and at least b (dB) at the frequency f4. With reference to FIG. 8, the attenuation of the receiving filter is smaller than b (dB) at the frequency f4 for other use, and there arises the problem that a sufficiently great S/N ratio is not available.
Accordingly with the conventional filter shown in FIG. 5, the bore 50 between the third electrode 32 and the conductive layer on the peripheral side surface and the conductive layer 39 near the opening of the bore are suitably adjusted in shape to thereby provide a trap having an attenuation pole, i.e., a great attenuation of c (dB), for example, in the vicinity of the frequency f3, and an attenuation of b (dB) also at a frequency over the frequency f4 shown in FIG. 7.
This method nevertheless requires an increased attenuation at f3, such that an attenuation occurs also near the frequency f2 within the required pass band as indicated at d. The same is also true of the transmission side including the first and second electrodes 30, 31.
In the field of mobile communications devices such as portable telephones, it is required in recent years that the parts be made ever smaller. Since mobile phones need to be highly portable as an important feature of the commercial product and must therefore be smaller in size, it is not desirable to provide improved filter characteristics by adding new circuit components to the dielectric filter described.
To overcome the foregoing problems, the present invention provides a dielectric filter device comprising a dielectric block generally in the form of a rectangular parallelepiped and having a plurality of hollow bores formed therein and openings of the bores in an upper surface of the block, electrically conductive layers respectively covering a lower surface opposed to the upper surface, an outer peripheral side surface parallel to axes of the bores, inner peripheral surfaces defining the respective bores and the upper surface around the bore openings, and a plurality of electrodes separate from the conductive layers for connection to external devices. First bore opening portions are arranged between the first electrode formed on the upper surface or side surface of the block and the second electrode formed on the upper surface thereof to provide a filter for passing a predetermined frequency band, second bore opening portions being arranged between the second electrode and the side surface of the block.
In the dielectric filter device described, the second bore opening portions are preferably arranged perpendicular to the direction of arrangement of the first bore opening portions.
The invention described gives improved characteristics to the dielectric filter without providing external components on the filter.